Directional valves are used in fluid circuits when the flows have to be divided or combined or, in different operational conditions, different branches of the fluid circuit are passed and other branches are excluded from passage or isolated.
Special requirements of the high-pressure and low-pressure resistance of such valves result for fluid circuits with difference pressure levels. An example of such a circuit is a combined cooling system/heat pump circuit, where, for example, with carbon dioxide as coolant, in one operational condition the coolant circuit is switched to act as cooling plant and in another operational condition the coolant, circuit is switched to act as heat pump. Dependent on the control state the coolant passes different branches of this coolant circuit with different pressure levels.
To make possible this multifunctionality of a coolant circuit for the above-mentioned cooling system and heat pump operational states, it is necessary to integrate directional valves into the coolant circuit. Such a directional valve is the sorting gate for the coolant flow, deciding whether the circuit is operated as cooling system or as heat pump.
In such known circuits a directional valve is positioned downstream of the compressor of the cooling system, which means that the directional valve must withstand the high-pressure and temperature levels of the coolant. Therefore special requirements of the pressure and temperature resistance of such a directional valve must be imposed. To close the circuit mentioned as an example also as a heat pump, another directional valve is required so that, after having passed the circuit, the coolant flows can be re-supplied to those components that are passed in both the heat pump and cooling plant operational modes.
In the state-of-the art WO 88/04745 a directional valve is disclosed that functions also at high temperatures and high fluid pressure. In this valve the high-pressure flow is directed within the valve body into different circuit branches dependent on the control position of a control element. The control position of the control element is controlled by a low-pressure control system with a spool and connected valve stems.
This directional valve is sealed to the outside by seals arranged around the valve stem. The control position of the valve and hence the function of the refrigerant circuit is determined by the low-pressure control system. To this end pressure is applied to the low-pressure control cylinder. The corresponding pressurized low-pressure control cylinder moves the spool, which is connected to the valve stem and the control element. On arrival at the control position, the control element opens a path for the refrigerant, and another path is closed.
It is a particular disadvantage of this system that an additional fluid system is required, namely the low-pressure control system for the movement of the control element to the control positions of the directional valve. The additional low-pressure control system further involves leakage points and hence sources of error of the total system. In addition, no rest position of the control element is defined. That means that in case of breakdown of the low-pressure control system the entire system breaks down, because the fluid circuit is not operable without the directional valve.
Further, in the state-of-the-art a directional valve is known by EP 1061314, which can be used for a combined cooling system/heat pump circuit.